I can vouch for Lara Croft GOL, fun in coop for sure. Portal 2 as well. RE5, not so much, I found I was more stressed out after playing than before, and I'm a low stress guy.
Alien Swarm is tough for 2 people, better with 4.

My buddy and I couldn't beat the last level on normal difficulty. Need at least 3.

It's actually quite easy to beat the game on insane as a solo Vegas tech, in fact it can be easier than a tag team on some levels. (you can either kill slowly or skip the monsters, your choice) Brutal/Onslaught is another story.

But yeah, if you two haven't done Portal2 or Alien Swarm yet, they're pretty fun.

EDIT: Forgot to mention, the custom maps for Alien Swarm are a lot of fun, only problem is you can't get them from Steam servers so finding people that have the map isn't easy. (which won't be a problem for a local co-op)